The present invention relates to a method and an apparatus for providing layers on the surfaces of workpieces, such as spotlight reflector inserts formed of plastic.
To coat a workpiece, such as a front surface mirror of a spotlight reflector insert, a vacuum chamber can be operated as a batching system, and a Plasma Chemical Vapor Deposition (PCVD) coating process, having a microwave Electron Cyclotron Resonance (ECR) plasma coating source, is used to coat the workpiece. The workpieces to be coated are secured to a rotary cage within the vacuum chamber. The rotary cage is conducted past the coating source with a frequency and phase-matched planetary motion.
German Patent DE 37 05 666 discusses an apparatus for producing a plasma and treating workpieces therein. A coating process used to coat a substrate, such as a band-shaped workpiece, is performed with the assistance of a microwave ECR plasma coating source.
German Patent DE 37 31 686 discusses a method and apparatus for depositing a corrosion-resistant layer onto the surfaces of lacquer-coated workpieces. A layer system is deposited onto the workpieces in a vacuum chamber with the assistance of a glow cathode and an evaporator.
In the above-discussed methods, the anti-corrosive layers are composed of a dielectric, or substantially dielectric material. Above thicknesses of approximately 100-200 nm, these layers are good insulators.
Traditional systems for applying protective coating layers to motor vehicle reflectors are operated using DC-PCVD technology. In using these systems, however, steadily growing layers form on the electrodes during the coating process. These layers reach a critical thickness for electrical conductivity within a short period of time, and thereafter, function as insulators. As a result of this layer growth, a glow discharge cannot be maintained to produce corrosion-resistant layers in a system operated with DC voltage. Further, as a result of the layer build-up, the electrodes must be cleaned, by flooding the process chamber, after each batch. However, flooding the process chamber is always accompanied by an enormous generation of fine dust particles, making it necessary to perform an involved cleaning of the entire process chamber. Thus, operation of a DC-PCVD system is extremely personnel intensive and cost-intensive. Further, DC-PCVD technology lacks in-line capabilities, as it has a limited attainable pressure range of a few mbars, and requires repeated cleaning of the process chamber.
One disadvantage of the DC-PCVD coating process is that the process cannot reliably manufacture layers exceeding approximately 100 nm. The DC glow discharge used in a DC-PCVD process is operated at a pressure range of a few mbars. Plasma flow at this pressure is highly viscous and turbulent, and thus, neither manageable nor controllable. As a result, it is difficult to provide a uniformly thick layer distribution within the operable pressure range.
A significant disadvantage of the DC-PCVD method is that the pre-lacquering layer has a lifetime of only a few hours, and thus, workpieces, such as base members of reflectors, that have been aluminized in one step will not adhere well to a subsequently applied protective-coating layer. Therefore, the workpieces must be further-processed shortly after applying the pre-lacquering layer. Because of the time dependence between the production steps of pre-lacquering and vacuum coating, storage between these steps is impossible.